Micro-machined thin film sensor arrays for the detection of H2, NH3, and sulfur containing gases, and method of making and using the same

ABSTRACT

The present invention provides a hydrogen sensor including a thin film sensor element formed by metal organic chemical vapor deposition (MOCVD) or physical vapor deposition (PVD), on a micro-hotplate structure. The thin film sensor element includes a film of a hydrogen-interactive metal film that reversibly interacts with hydrogen to provide a correspondingly altered response characteristic, such as optical transmissivity, electrical conductance, electrical resistance, electrical capacitance, magneto resistance, photoconductivity, etc., relative to the response characteristic of the film in the absence of hydrogen. The hydrogen-interactive metal film may be overcoated with a thin film hydrogen-permeable barrier layer to protect the hydrogen-interactive film from deleterious interaction with non-hydrogen species. The hydrogen permeable barrier may comprise species to scavenge oxygen and other like species. The hydrogen sensor of the invention may be usefully employed for the detection of hydrogen in an environment susceptible to the incursion or generation of hydrogen and may be conveniently configured as a hand-held apparatus.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part application to U.S.patent application, Ser. No. 09/231,277, filed on Jan. 15, 1999,entitled “MICRO-MACHINED THIN FILM HYDROGEN GAS SENSOR, AND METHOD OFMAKING AND USING THE SAME.” Additionally, this patent application herebyincorporates by reference U.S. patent application, Ser. No. 09/231,277,filed on Jan. 15, 1999, entitled “MICRO-MACHINED THIN FILM HYDROGEN GASSENSOR, AND METHOD OF MAKING AND USING THE SAME.” The disclosures of theforegoing references are hereby incorporated herein by reference intheir entireties, together with the disclosures of the following pendingUnited States Patent Applications: U.S. patent application Ser. No.09/042,698 filed Mar. 17, 1998 in the names of Gautam Bhandari andThomas H. Baum for “Hydrogen Sensor Utilizing Rare Earth Metal Thin FilmDetection Element, and Differential Optical Sensing Method for Detectionof Hydrogen” and U.S. patent application Ser. No. 09/081,957 filed May19, 1998 in the name of Glenn M. Tom for “Piezoelectric Quartz CrystalHydrogen Sensor, and Hydrogen Sensing Method Utilizing Same.”

FIELD OF THE INVENTION

[0002] The present invention relates to a micro-machined thin film gassensor device, and a method of making and using the same. Morespecifically, the present invention relates to solid-state sensor arraysfor the detection of H₂, NH₃, and sulfur containing gases.

BACKGROUND OF THE INVENTION

[0003] Hydrogen gas is used in variety of applications ranging fromsemiconductor thin film processing to rocket fuel in the aerospaceindustry. The combustible nature of hydrogen however, makes itsdetection vitally important. A common need in each of these and othersimilar technologies is the ability to detect and monitor gaseoushydrogen. Hydrogen gas sensors that quickly and reliably detect hydrogenover a wide range of oxygen and moisture concentrations are notcurrently available, and must be developed in order to facilitate thetransition to a hydrogen based energy economy

[0004] “Hydrogen will join electricity in the 21^(st) Century as aprimary energy carrier in the nation's sustainable energy future.” (DOE1995) This bold statement was made as part of the 1995 Hydrogen Visionand reflects the tremendous potential of hydrogen as an energy system.The abundance and versatility of hydrogen suggests that it can providesolutions to problems encountered with current fossil fuel energysystems, such as declining domestic supplies, air pollution, globalwarming, and national security.

[0005] Significant research and development efforts are currentlyunderway to make the widespread use of hydrogen technically andeconomically feasible. These efforts are directed toward creating thebasic building blocks of a hydrogen economy: production, storage,transport and utilization. An underlying need of each of these buildingblocks is the ability to detect and quantify the amount of hydrogen gaspresent. This is not only required for health and safety reasons, butwill be required as a means of monitoring hydrogen based processes. Forexample, if hydrogen were to be introduced as an automobile fueladditive, several sensors would be needed to detect potential hydrogengas leaks, as well as to monitor and provide feedback to regulate theair/fuel/hydrogen mixture.

[0006] Although the safety record of the commercial hydrogen industryhas been excellent, it is estimated that undetected leaks were involvedin 40% of industrial hydrogen incidents that did occur. Emerginghydrogen based energy systems will require hydrogen sensors that are asubiquitous as computer chips have become in our factories, homes, and inour cars. This means that the ability to produce large volumes ofsensors at a low cost is paramount.

[0007] In order to support an effective hydrogen detection andmonitoring system, the hydrogen sensor element must fulfill severalrequirements. It needs to be selective to hydrogen in variety ofatmospheres (including the oxygen-rich high-humidity environments foundin fuel cells). It must have a good signal to noise ratio and a largedynamic range. Speed of detection is a critical requirement to ensurerapid response to potentially hazardous leaks. Long lifetimes betweencalibrations are desirable in order to minimize maintenance. Low powerconsumption is requisite for use in portable instrumentation andpersonnel monitoring devices. Ultimately, these must all be achieved bya safe sensor element that is affordable to manufacture in largenumbers, so that safe design principles are the deciding factor in thenumber and locations of detection points.

[0008] Existing gas sensors are not adequate, either technically and/orfrom a cost standpoint. Issues such as size, thermal range, and lifetimehave proven to be substantial hurdles for current technologies toovercome. Therefore, there is a need to address these issues, with thedevelopment of MEMS (Micro-Electro-Mechanical Systems) based solid-statesensor arrays for the detection of, NH₃, and sulfur containing gases.

[0009] Additionally, hydrogen-containing gases such as CH₄, C₂H₆,acetone, methanol etc. are used in large variety of industrialapplications ranging from semiconductor thin film processing topetroleum and polymer manufacturing. The combustible nature of many ofthese gases as well as the always-increasing need for improved processcontrol makes the detection and monitoring of these gases vitallyimportant. Difficulties with the sensors that are currently used todetect these gases are that they are not chemically specific, and oftenwill have similar response for different gases. In addition, many ofthese sensors are combustion based and rely on the presence of oxygen.Therefore, a need exists for a sensor with reproducible results specificto individual hydrogen containing gases, able to operate in environmentswith little to no oxygen present. Furthermore, a need exists for a gassensor having no moving parts, a response time on the order of seconds,with minimal power consumption, and capable of being used in a hand heldportable instrument

[0010] About one-half of all the sensors used to measure hazardous gasesmeasure hydrogen. The bulk of these systems utilize as the detectorelement a Group VIIIB metal element (Ni, Pd, Pt) that is heated tocatalytically oxidize the hydrogen, with the resulting change in heatload being the measured parameter for determination of the presence ofhydrogen.

[0011] Sensors of such “hot wire” type have cross-sensitivity to othereasily oxidized materials, such as alcohols and hydrocarbons. Sucheasily oxidized materials are common components of gases in asemiconductor-manufacturing environment, and in such application theresult is frequent occurrence of false alarms.

[0012] Since the current generation of hot wire sensors require anoxidation reaction for operation, such sensors are unable to detecthydrogen when it is present in inert gas streams or environments, whichare not of a character to support oxidative reaction. This is a severedeficiency of such hot wire sensors and limits their applicability andutility.

[0013] It would be a significant advance in the art to provide a sensorovercoming the aforementioned deficiencies of current hot wire sensors.

[0014] Another class of sensors includes metal-insulator semiconductor(MIS) or metal-oxide-semiconductor (MOS) capacitors and field effecttransistors, as well as palladium-gated diodes. In general however,these sensors are limited to detecting low concentrations of hydrogen.

[0015] Because hydrogen is used in such a wide variety of environments,it is desirable to have a sensor that will be reproducible and specificto hydrogen, even with varying concentration of background gases such asoxygen, water and other contaminants.

[0016] It is also desirable to have a solid state sensor that has nomoving parts, has a response time on the order of seconds, would operatewith minimum power consumption, does not require frequent calibration,and could be used in a hand-held portable instrument.

[0017] It therefore is one object of the present invention to provide animproved hydrogen sensor.

[0018] It is another object of the invention to provide a hydrogensensor that senses the presence of hydrogen in a reproducible andhydrogen-specific manner.

[0019] It is another object of the invention to provide a hydrogensensor that senses the presence of hydrogen in a reproducible andhydrogen-specific manner, even with varying concentration of backgroundgases such as oxygen, water and other contaminants.

[0020] It is yet another object of the present invention to provide asolid state hydrogen sensor that has no moving parts, has a responsetime on the order of seconds, operates with minimum power consumption,does not require frequent calibration, has a large dynamic detectionrange, and can be readily embodied as a hand-held portable instrument.

[0021] Other objects and advantages of the present invention will bemore fully apparent from the ensuing disclosure and appended claims.

SUMMARY OF THE INVENTION

[0022] The present invention relates in one aspect to a hydrogen sensor,comprising a hydrogen-interactive thin film sensor element on amicro-hotplate structure.

[0023] The hydrogen-interactive thin film sensor element of such sensormay comprise a hydrogen-interactive thin film (i) arranged for exposureto an environment susceptible to the incursion or generation of hydrogenand (ii) exhibiting a detectable change of physical property when thehydrogen-interactive thin film is exposed to hydrogen. Such detectablechange of physical property may comprise optical transmissivity,electrical resistivity, electrical conductivity, electrical capacitance,magneto-resistance, photoconductivity, and/or any other detectableproperty change accompanying the exposure of the thin film sensorelement to hydrogen. The hydrogen sensor may further include a detectorconstructed and arranged to convert the detectable change of physicalproperty to a perceivable output, e.g., a visual output, auditoryoutput, tactile output, and/or auditory output.

[0024] In one embodiment of the present invention these sensors couplenovel thin films as the active layer with a MEMS structure known as aMicro-Hotplate. This coupling results in a H₂ gas sensor that hasseveral unique advantages in terms of speed, sensitivity, stability andamenability to large-scale manufacture. Results indicate that thistechnology has substantial potential for meeting the sensingrequirements of a hydrogen based energy economy.

[0025] In another embodiment, the hydrogen-interactive thin film isoverlaid by a hydrogen-permeable material protecting the rare earthmetal thin film from deleterious interaction with non-hydrogencomponents of the environment being monitored, such as nitrogen, oxygen,ammonia, hydrocarbons, etc. The protective-over layer may include ametal such as Pd, Pt, Ir, Rh, Ag, Au, Co, and/or alloys thereof.However, these alloys may not be sufficient to prevent the rare earthfilms and their alloys from oxidizing over long time periods. There areseveral possible mechanisms for this. Oxygen and other like species candiffuse through the rare earth film or film grain boundaries. If this isthe case, a thicker rare earth coating may represent a viable solution.However, this solution may result in decreased sensitivity andresponsivity. Another possibility is that the rare earth film isembrittled with repeated hydrogen exposure, and failing with time due tothe α to β phase transformation of rare earth hydrides that occurs at ahydrogen concentration of −2%. One solution is to suppress this phasetransition in rare earth films such as palladium by alloying the rareearth film with suitable elements, such as silver, titanium, nickel,chromium, aluminum or other species known to those skilled in the art.In this manner, the long-term stability of the underlying coating can beimproved.

[0026] The micro-hotplate structure in the sensor of the invention maybe advantageously constructed and arranged for selectively heating thehydrogen-interactive thin film gas sensor element according to apredetermined time-temperature program, e.g., involving cyclic heatingof the hydrogen-interactive thin film gas sensor element by themicro-hotplate structure.

[0027] The invention relates in another aspect to a hydrogen sensordevice, comprising:

[0028] a micro-hotplate structure;

[0029] a hydrogen-interactive thin film gas sensor element on themicro-hotplate structure; and

[0030] a detector for sensing a detectable change of physical propertyof the film in exposure to hydrogen and generating a correlative outputindicative of hydrogen presence.

[0031] A power supply may be provided in such device and may beconstructed and arranged for actuating the micro-hotplate structureduring and/or subsequent to sensing the detectable change of physicalproperty of the rare earth metal thin film in exposure to hydrogen,and/or for energizing the detector.

[0032] A further aspect of the invention relates to a method offabricating a hydrogen sensor on a substrate, comprising:

[0033] constructing on the substrate a micro-hotplate structure; and

[0034] forming on the micro-hotplate structure a hydrogen-interactivethin film that in exposure to hydrogen exhibits a detectable change ofat least one physical property, and wherein the hydrogen-interactivethin film is arranged to be heated by the micro-hotplate structure.

[0035] A still further aspect of the invention relates to a method ofdetecting hydrogen in an environment, comprising:

[0036] providing a hydrogen sensor device comprising ahydrogen-interactive thin film operatively coupled with a micro-hotplatestructure for selective heating of the hydrogen-interactive thin film,with the hydrogen-interactive thin film being arranged for exposure tothe environment and exhibiting a detectable change of physical propertywhen the hydrogen-interactive thin film is exposed to hydrogen;

[0037] exposing the hydrogen-interactive thin film to the environment;

[0038] outputting said detectable change of physical property when thepresence of hydrogen in the environment is detected; and

[0039] selectively heating the hydrogen-interactive thin film by themicro-hotplate structure during and/or subsequent to detection ofhydrogen in said environment, to enhance the performance of thehydrogen-interactive thin film for detection of hydrogen.

[0040] Other objects and advantages of the invention will be more fullyapparent from the ensuing disclosure and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings in whichlike reference numerals indicate like features and wherein:

[0042]FIG. 1 plots long-term response behavior to hydrogen exposures asa function of time. Between each response shown was a continuousprocession of similar exposures on a 50-minute period;

[0043]FIG. 1 plots the magnitude of response to hydrogen exposure as afunction of date, from the data in FIG. 1. Both the fractional andderivative strength are plotted. Between each data point, the film wascontinuously exposed to hydrogen concentrations on a 50-minute period;

[0044]FIG. 3 is a scanning electron microscope (SEM) micrograph of athin film sensor including a thin film sensor element deposited by metalorganic chemical vapor deposition (MOCVD) on a micro-hotplate structure;

[0045]FIG. 4 is an exploded view of constituent layers of a hydrogensensor according to one embodiment of the present invention;

[0046]FIG. 5 is a schematic cross-sectional elevation view of a hydrogensensor according to one embodiment of the present invention showing theconstituent layers of the structure on a silicon substrate;

[0047]FIG. 6 is a schematic representation of a hydrogen sensorapparatus according to one embodiment of the invention;

[0048]FIG. 7 is a perspective view of a hand-held hydrogen sensorapparatus according to one embodiment of the present invention;

[0049]FIG. 8 is a graph showing the resistance response of apalladium/yttrium micro-hotplate sensor as a function of time whenexposed to various concentrations of hydrogen in a background gas of 1atmosphere of nitrogen;

[0050]FIG. 9 is a graph showing the resistance response of apalladium/yttrium micro-hotplate sensor as a function of hydrogenconcentration, for hydrogen exposures carried out in a background gas of1 atmosphere of nitrogen.

[0051]FIG. 10 plots the Resistive response of a micro hotplate based H₂gas sensor to repeated exposure to 0.25% H₂ in air. The magnitude ofresponse is greater than 120% of the pre-exposure baseline;

[0052]FIG. 11 provides an expanded scale plot of the resistive responseof a micro hotplate based H₂ gas sensor to exposure to 0.25% H₂ in air,with a demonstrated speed of response <0.5 sec;

[0053]FIG. 12 plots the resistive response of a micro hotplate based H₂gas sensor to concentrations of H₂ in air ranging from 1% to 0.01%;

[0054]FIG. 13 plots the resistive response from the previous FIGURE,plotted as function of H₂ gas concentration; and

[0055]FIG. 14 plots stability results over a 7-day time frame;

[0056]FIG. 15 provides a schematic diagram of platinum islands onpalladium coated metal hydride active layer. In this diagram, thedisassociation of water is used as an example; and

[0057]FIG. 16 provides a schematic diagram of separate catalytic andsensing hotplate devices operated in tandem to detect catalyticallydisassociated hydrogen for CH₄ as an example.

DETAILED DESCRIPTION OF THE INVENTION

[0058] Embodiments of the present invention are illustrated in theFIGUREs, like numerals being used to refer to like and correspondingparts of the various drawings.

[0059] The present invention provides a hydrogen sensor including a thinfilm sensor element formed by metal organic chemical vapor deposition(MOCVD) or physical vapor deposition (PVD), on a micro-hotplatestructure. The thin film sensor element includes a film of ahydrogen-interactive metal film that reversibly interacts with hydrogento provide a correspondingly altered response characteristic, such asoptical transmissivity, electrical conductance, electrical resistance,electrical capacitance, magneto resistance, photoconductivity, etc.,relative to the response characteristic of the film in the absence ofhydrogen. The hydrogen-interactive metal film may be overcoated with athin film hydrogen-permeable barrier layer to protect thehydrogen-interactive film from deleterious interaction with non-hydrogenspecies. The hydrogen permeable barrier may comprise species to scavengeoxygen and other like species. The hydrogen sensor of the invention maybe usefully employed for the detection of hydrogen in an environmentsusceptible to the incursion or generation of hydrogen and may beconveniently configured as a hand-held apparatus.

[0060] Stability is an important consideration in all sensor developmentefforts, and understanding the source and mechanism of instabilities anddrift is vital for the development of useful devices. Through the usefullife of many devices, decreases in the responsivity of the thin filmdevices are observed over time. This is illustrated in FIG. 1, whichshows the response of a palladium coated yttrium film deposited on aminiature light bulb, taken over the coarse on one month.

[0061] Between recorded observations, the film was continuouslysubjected to repeated hydrogen exposures on a 50-minute interval. Thisdata was collected with a sensor system that utilizes a silicon photodiode. Over time, there is a monotonic increase in the baselinetransmissivity as well as a decrease in the magnitude of the response asa function of hydrogen concentration. FIG. 2 quantifies thoseobservations and plots the magnitude of response to 0.78% hydrogenexposure in air as a function of measurement date. The magnitude ofresponse was calculated as the fraction of the base line, and as thestrength of the derivative. (Both of these measurements are required forthe accurate detection of hydrogen, as the derivative provides responsesto rapid changes and concentration, and the change with respect tobaseline would be required to detect slower concentration build ups.)Although the response for this sample has fallen off by a factor of 2 to3 over the course of ˜1 month, it appears that this decrease has leveledoff and that there is still sufficient signal to noise present.

[0062] The present invention relates to a hydrogen sensor integrating athin film hydrogen sensor element with a micro-hotplate structure. Thehydrogen sensor of the invention is a solid-state device that may beadapted in a variety of apparatus embodiments to accommodate the objectsof the invention. In one embodiment, the hydrogen-interactive thin filmis overlaid by a hydrogen-permeable material protecting the rare earthmetal thin film from deleterious interaction with non-hydrogencomponents of the environment being monitored, such as nitrogen, oxygen,ammonia, hydrocarbons, etc. The protective-over layer may include ametal such as Pd, Pt, Ir, Rh, Ag, Au, Co, and/or alloys thereof.However, these alloys may not be sufficient to prevent the rare earthfilms and their alloys from oxidizing over long time periods. Onesolution provided by the present invention is to suppress this phasetransition in rare earth films such as palladium by alloying the rareearth film with suitable elements, such as silver, titanium, nickel,chromium, aluminum or other species known to those skilled in the art.In this manner, the long-term stability of the underlying coating can beimproved.

[0063] In the embodiment illustrated in FIG. 3, the rare earth filmcomprises a palladium alloy formed using single source e-beam depositiontechniques. A 200-nm thick Y film 10 coated with a second film 12comprising 120 nm of Pd and 30 nm of Ag at room temperature. Thepalladium was deposited in three steps, alternating with equal amountsof silver These films were subsequently annealed at differenttemperatures, and x-ray diffraction indicates that alloying occur attemperatures over 250° C. It seems that there is a significant thin filmeffect that results in alloying at temperatures that are well below themelting points of either metal.

[0064] The sensing mechanism of the hydrogen sensor device of thepresent invention is based on the reversible, hydrogen-inducedtransition from the metallic di-hydride compound to the semi-conductingtri-hydride compound, according to the following equation:${MH}_{2}\quad {({metallic})\overset{{1/2}\quad H_{2}}{}\quad {MH}_{3}}\quad ({semiconducting})$

[0065] wherein M comprises the hydrogen-interactive thin film element.The hydrogen-interactive thin film element may comprise one or more thinfilms wherein at least one thin film is selected from the groupconsisting of rare earth metals, Group II elements or any combinationthereof. The rare earth metal and the Group II element may be combinedto form a Group II element doped rare earth metal thin film or an alloythin film comprising the rare earth metal and the Group II element. Rareearth and alkaline earth hydride films are extremely oxophilic innature, and may also interact with other atmospheric or environmentalspecies in a manner that masks the hydrogen interaction. In order toobviate such deleterious interactions with non-hydrogen species, wherethe hydrogen sensor is intended to operate in environments containingsame, it may be advantageous to overcoat the hydrogen-interactive thinfilm of the sensor with a protective film layer of a coating that ispermeable to hydrogen, but is impermeable to the deleterious interactionspecies present in the environment. One such protective film layermaterial is palladium (Pd). Hydrogen is known to diffuse readily througha Pd film, while oxygen and nitrogen do not readily penetrate the Pdfilm, thus allowing the formation of the rare earth metal and/or GroupII hydride without the formation of oxides and/or nitrides. Furthermore,it may be advantageous to incorporate scavenging components within theprotective film layer 12 in order to obviate such deleteriousinteractions with non-hydrogen species. In certain embodiments theseefforts may be directed to scavenging oxygen and other like speciesknown to those skilled in the art.

[0066] By way of specific example, in sensor devices constructed inaccordance with the invention, including an yttrium (Y) sensor film 10overcoated with a Pd film layer 12, the sensor film 10 was found to besensitive to hydrogen in a nitrogen environment, to hydrogen in apentane environment, and to hydrogen in an ammonia environment, thusdemonstrating the selectivity of the sensing film in such environments.

[0067] The integration of such hydrogen-interactive sensor films withmicro-hotplate structures in accordance with the present inventionpermits the selective heating of the sensor film by the micro-hotplatestructure, thereby increasing the rate of interaction of the sensor filmwith any hydrogen gas in the environment being monitored, as well asincreasing the rate of regeneration or recovery of the sensor film.Thus, the sensor film may be selectively heated during the activesensing operation so that the reaction of YH₂+½H₂→YH₃ is increased, tothereby enhance the sensitivity of the hydrogen sensor device, and afterthe sensing is complete, the sensor film may be further heated to highertemperature to cause the reverse reaction YH₃→YH₂+½H₂ to take place. Themicro-hotplate may therefore be coupled with suitable power supply andcycle time controller circuitry, so that the micro-hotplate structureprovides appropriate heating of the hydrogen-interactive sensor film forthe desired monitoring operation. Such power supply and cycle timecontroller circuitry may for example be constructed and arranged forpulsed or variable cycle time operation, or according to a selectedtime-temperature schedule.

[0068] Such micro-hotplate structure heating of the hydrogen sensor filmsignificantly enhances the operation of the sensor device of theinvention, relative to a corresponding sensor device lacking themicro-hotplate structure. For example, in a sensor device lacking themicro-hotplate structure, for ambient temperature sensing of hydrogengas, typical response times were on the order of 1 minute after exposureto H₂, but complete recovery after removal of the H₂ source from thesensor was on the order of hours. By contrast, heating of the sensorfilm by the micro-hotplate structure substantially improves both theresponse and recovery times of the sensor device. The micro-hotplateallows electrical measurement of the sensor film while controlling thetemperature of the film, thus allowing the formation of the hydride in ahighly effective manner.

[0069] The hydrogen-interactive sensor film may be readily formed on themicro-hotplate by any suitable deposition techniques, such as, forexample, sputter deposition, solution deposition, metal-organic chemicalvapor deposition (MOCVD), physical vapor deposition (PVD), andcorresponding assisted vapor deposition processes, such asplasma-assisted MOCVD.

[0070] The technique for forming the hydrogen-interactive sensor film onthe micro-hotplate structure is by physical vapor deposition or chemicalvapor deposition. If CVD is employed, then the individualmicro-hotplates can be separately heated, in a self-lithographic processflow.

[0071] The sensors of the present invention couple novel thin films asthe active layer with a MEMS structure known as a Micro-Hotplate. Thiscoupling results in a H₂ gas sensor with several unique advantages interms of speed, sensitivity, stability and amenability to large-scalemanufacture. Some embodiments of the present invention have demonstratedresponse speeds of <0.5 s to 1% H₂ in dry air, and the ability to detect<200 ppm. Additionally, the system and method of the present inventioncan be readily and inexpensively produced at large quantities.

[0072] The micro-hotplate structure of the sensor device of theinvention may be readily fabricated by micro-machining techniques, asfor example based on complementary metal oxide semiconductor (CMOS)fabrication techniques.

[0073] One illustrative embodiment of sensor fabrication involves thefollowing steps. A desired micro-hotplate array is designed and laidout, and may for example comprise 4, 8 or more individual micro-hotplateelements. This micro-hotplate array can then be fabricated in acommercial CMOS process using a facility such as the MOSIS system. Theresulting micro-hotplate array is micro-machined and packaged. Next, thepackaged chip can be placed in either a PVD or a CVD chamber and atleast one thin metallic film of the hydrogen-interactive film materialcan be deposited on the hotplate elements of the micro-hotplatestructure. With the appropriate electrical feed-throughs, the hotplateelements can be heated to improve the properties of the metal filmdeposition. Additionally, with appropriate electrical feed-throughs, theresistance of the deposited films can be monitored in situ and used as afeedback variable for the deposition process. For example, when aspecific value of conductance is reached, the film will have aparticular thickness, and the conductance value can be utilized forcontrol purposes in the film formation step, to stop the film growthoperation at the point that the deposited film reaches the desiredthickness. This feedback deposition technique can be used for each ofthe hydrogen-interactive film and the optional protective over layerfilm of hydrogen-permeable, extraneous species-impermeable material, toachieve a desired film thickness of each such layer of the sensorelement.

[0074] Another embodiment would follow the same basic steps as describedabove, but with the thin metallic film of the hydrogen-interactive filmmaterial deposited on the hotplate elements of the micro-hotplatestructure before micro-machining and packaging

[0075] Another embodiment would follow the same basic steps as describedabove with the exception that an alternative process might be used tofabricate the micro-hotplate structure instead of the CMOS process. Suchalternative process might substitute Pt or W for the Al metallizationtypically used in the CMOS process. In any of such embodiments, both thehydrogen-interactive film and the optional protective-over layer filmmay be made of different thicknesses within the same array (overdifferent ones of the multiple micro-hotplate elements) to cover abroader dynamic range of hydrogen detection capability. For example, athinner protective- over layer film of Pd on the hydrogen-interactivesensor film can be used to detect a lower concentration of hydrogen,while a thicker protective-over layer film of Pd on thehydrogen-interactive sensor film can be used to detect a higher hydrogenconcentration, since a higher concentration driving force is requiredfor the diffusion of hydrogen through the thicker protective-over layerfilm to occur, relative to the diffusion of hydrogen gas through athinner protective-over layer film.

[0076] The optimal operation temperature or temperatures of the hydrogensensors of the invention may be readily empirically determined withoutundue experimentation, for specific sensing applications.

[0077] As a consequence of the rapid thermal rise and thermal fall timesthat are characteristic of temperatures for micro-hotplate operation,pulsed temperature operation can be advantageously employed in use ofthe hydrogen sensor device of the invention. For example, as alluded tohereinabove, the hydrogen interactive sensor films may be most sensitiveto initial hydrogen exposure at one specific temperature, but require ahigher temperature to be returned to their initial state (for subsequentactive sensing operation). In such instance, it may be desirable topulse the micro-hotplate periodically to refresh thehydrogen-interactive sensor film, thereby minimizing the effect of driftand improving long term stability of the device.

[0078] The present invention thus makes use of the fact that uponexposure to hydrogen, hydrogen-interactive thin films exhibit strikingchanges in physical properties, changing from metallic (conducting) tosemiconducting phases. These phase changes are accompanied by changes inelectrical resistivity, magneto-resistance and photoconductivity of thehydrogenated rare earth thin film.

[0079] The invention contemplates a wide variety of sensor devices andapparatus, as well as methodology, which utilize hydrogen-interactivethin films with which hydrogen is interactive to produce both a physicaland chemical change in the properties of the hydrogen-interactive thinfilm.

[0080] In the practice of the invention, as described brieflyhereinabove, the hydrogen-interactive thin film is overlaid by aprotective-over layer which is hydrogen-permeable, but which is at leasthighly impermeable to reactive species that could otherwisedeleteriously interact with the rare earth metal thin film and preventit from producing the desired physical property change of the filmincident to exposure of the film to hydrogen. These alloys may suppressthis phase transition in rare earth films such as palladium by alloyingwith suitable elements, such as silver, titanium, nickel, chromium,aluminum or other species known to those skilled in the art. In thismanner, the long-term stability of the underlying coating can beimproved. Reducing percentage Ag alloying to 10% slowed the recoveryrate. Reducing the coating thickness and percent Ag alloying are methodsto improve the recovery rate of the Ag alloyed Pd films. Reducing thetotal capping film thickness to about 15 nm (12 nm Pd and 3 nm Ag)improved the sensor's reaction and recovery rates. While reducing the Agto 10% significantly degraded the sensor's performance The palladiumthin film may need more than 10% Ag alloy to be fully stabilized.

[0081] The gradual oxidation of the rare earth sensing film 10 isresponsible for long term signal shift, and this oxidation isexacerbated by repeated cycling of the palladium from the α to β hydridephases. Consequently, the present invention focuses on the optimizationof the noble metal capping layer 12 through alloying, as this phasechange in palladium can be suppressed by alloying with silver, chromium,nickel, titanium or other like metals as known to those skilled in theart. Several experimental factors as listed in Table 1 may be altered toinvestigate a number of compositions and their effect of sensorstability. Factors effecting sensor stability Active Layer PrimaryElement Active Layer Dep. Temp. Active Layer Thickness Active LayerDeposition Rate Capping Layer Primary Element Capping Layer DepositionTemperature Capping Layer Dopant/Alloy Capping Layer Deposition Rate %Alloy Total barrier layer thickness Number of layers

[0082] As used herein, the term “hydrogen-interactive thin film element”means one or more thin films wherein at least one thin film is selectedfrom the group consisting of one or more rare earth metals, one or moreGroup II elements as well as alloys or combinations thereof. As usedherein the term “rare earth metal means a metal selected from scandium,yttrium, lanthanum, the lanthanides, and the actinides as well as alloysand combinations of such metals, and alloys and combinations of suchmetals with Group II elements, e.g., calcium, barium, strontium,magnesium and radium. The lanthanides are the 14 elements followinglanthanum in the Periodic Table, viz., cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium and lutetium. The actinides are theelements of the Periodic Table having the atomic numbers 89 through 103inclusive, viz., actinium, thorium, protactinium, uranium, neptunium,plutonium, americium, curium, berkelium, californium, einsteinium,fermium, mendelevium, nobelium and lawrencium.

[0083] The physical property of the hydrogen-interactive thin film thatis altered in response to the presence of hydrogen may be the opticaltransmissivity of the film to optical radiation incident on the sensorelement, electrical resistivity, electrical conductivity, magnetoresistance, photoconductivity, electrical capacitance, or any otherphysical and/or chemical properties that are changed in exposure of thehydrogen-interactive thin film to hydrogen. Appropriate detector andoutput components, to provide an output indicative of the presence ofhydrogen in the environment to which the hydrogen-interactive thin filmof the sensor is exposed, readily monitor the change in physicalproperty of the hydrogen-interactive thin film.

[0084] The aforementioned changes in properties of hydrogen-interactivethin films, incident to their exposure to hydrogen, result from achemical equilibrium between the dihydride and trihydride forms of suchfilms. When hydrogen is present, a dynamic equilibrium exists betweenthe two forms and the physical and optical changes can be quitedramatic.

[0085] For example, in the presence of hydrogen, noble metal (e.g., Pd,Pt) overcoated Y reacts to form the dihydride (YH₂). Further exposure tohydrogen results in the formation of the trihydride YH₃. This secondstep occurs at room temperature (e.g., about 25 degrees Centigrade) andambient pressure (e.g., about 1 atmosphere) and is completelyreversible. The formation of YH₂, on the other hand, is essentiallyirreversible at room temperature and ambient pressure, as a result ofits relatively large heat of formation (−114 kJ/mol H) compared with theequilibrium step (−41.8 kJ/mol H or −44.9 kJ/mol H). This process isillustrated in the following formula:$\left. {Y + H_{2}}\rightarrow{{YH}_{2}\overset{{1/2}\quad H_{2}}{\rightarrow}{YH}_{3}} \right.$

[0086] The transition of the optically reflecting rare earth dihydrideto the optically transparent rare earth trihydride is a chemical changewith electronic origins. The dark blue reflecting phase of YH₂ ismetallic, whereas the transparent phase (YH₃) is semiconducting with adirect band gap of 1.8 eV. This change of state—from metallic tosemiconducting—can therefore be readily quantified by measuring theresistance of the film under hydrogen exposure conditions. Resistancemeasurements allow the correlation of the optical and electricalbehavior of the films.

[0087] As a consequence of the ability of micro-hotplates to localizehigh temperature heating to microscopic regions of the device structure,the sensors of the present invention can utilize elevated temperaturesto enhance the hydrogen sensing operation without the dangers ofhydrogen ignition that have plagued the prior art “hot wire” sensorsdescribed in the Background section hereof.

[0088] Further, the temperature control capability of the micro-hotplatestructure permits the thermal management of the sensor in a highlyeffective and efficient manner. Qualitatively the rare earth dihydrideto trihydride transition is an exothermic chemical reaction (negativeAG: −41.8 kJ/mol H or −44.9 kJ/mol H). Thus, the micro-hotplatestructure can be selectively actuated and controlled to provideappropriate temperatures favorable to hydrogen gas sensing.

[0089] While we do not wish to be bound by any theory as regards thespecific mode or mechanism of behavior of the rare earth thin filmsensors in accordance with the present invention, it is believed that ametal-insulator transition rather than a structural phase change causesthe observed physical properties transformation.

[0090] The selectivity exhibited by hydrogen-interactive thin filmsallows, for the first time, fabrication of inexpensive hydrogen sensorsthat can be deployed in large numbers to remotely monitor hydrogenlevels over large areas. Furthermore, hydrogen-interactive thin filmscan operate in an industrial or manufacturing environment containingtrace organic vapors. We are not aware of any existing hydrogen sensingtechnologies having these attributes.

[0091] Hydrogen-interactive thin films can be coated with materials suchas palladium or platinum to provide an effective barrier to oxidation,yet enable hydrogen to diffuse through to the rare earth thin film,thereby acting as a selective membrane for hydrogen in the sensorelement.

[0092] The deposition of hydrogen-interactive thin films on themicro-hotplate substrate may be readily carried out using at least oneorganometallic precursor of the rare earth metal or the Group II elementthat thermally decomposes to the metal hydride or elemental metal in areducing environment of hydrogen. Under some conditions, the directformation of rare earth metal hydride materials may be realized.

[0093] The invention enables a hydrogen detection system to beconstructed for monitoring an extended or remote area region for theincursion or generation of hydrogen therein. The hydrogen detectionsystem may include a multiplicity of rare earth metal thinfilm/micro-hotplate detector devices each of which (i) is arranged forexposure to a specific individual locus of the extended area region and(ii) exhibits a detectable change of physical property, e.g., opticaltransmissivity, electrical resistivity, electrical conductivity,electrical capacitance, magneto-resistance and/or photoconductivity,when the hydrogen-interactive thin film of the detector device iscontacted with hydrogen gas at such locus.

[0094] The hydrogen detection system described in the precedingparagraph may be constructed and arranged so that different physicalproperties are detected when multiple detector devices are contactedwith hydrogen gas at different loci of the extended area region.

[0095] The hydrogen sensor of the invention is readily fabricated byforming on the micro-hotplate substrate a hydrogen-interactive thin filmthat is responsive to contact with hydrogen by exhibiting a detectablechange of physical property, and coupling the thin film with means forexhibiting the detectable change of physical property when thehydrogen-interactive thin film is exposed to hydrogen.

[0096] The means for exhibiting the detectable change of physicalproperty when the hydrogen-interactive thin film is contacted withhydrogen gas, may for example comprise a colored substrate, whereby thedetectable change of physical property entails a change from opacity totransparency when the hydrogen-interactive film is contacted withhydrogen gas or a change in color as determined by the colored layer inclose proximity to the hydrogen sensitive layer (lanthanum hydride film)in its transmissive form. By such arrangement, the colored substrate isobscured in the absence of hydrogen, and rendered visible when hydrogenis present and converts the formerly opaque film to a transparent film.

[0097] The means for exhibiting the detectable change of physicalproperty when the hydrogen-interactive thin film is contacted withhydrogen gas may include suitable circuit means for signal processingthe change of physical property and generating an output indicative ofthe presence or absence of hydrogen gas.

[0098] In the practice of the invention, the hydrogen-interactive thinfilm is formed on the substrate by a technique such as physical vapordeposition, chemical vapor deposition, sputtering, solution deposition,focused ion beam deposition, electrolytic plating, or electrolessplating. The hydrogen-interactive thin film may also be separately bothmetal dihydride and metal trihydride reaction products, wherein themetal dihydride and metal trihydride reaction products have differingphysical properties. The physical property change may for exampleinclude an optical transmissivity change, such as a change of opticalopacity to optical transparency and discretely formed as an independentelement, remotely from the micro-hotplate structure, and subsequentlysecured or placed on the micro-hotplate structure, to form theintegrated sensor.

[0099] Most preferably, the hydrogen-interactive thin film is formed onthe substrate by physical vapor deposition, or alternatively by chemicalvapor deposition, e.g., by liquid delivery chemical vapor deposition,using an organometallic precursor that thermally decomposes to the metalhydride or elemental metal in a reducing environment of hydrogen.

[0100] The hydrogen-interactive thin film in the sensor may in oneembodiment comprise a rare earth metal thin-film. The rare earth metalthin film may include a rare earth metal component such as a trivalentrare earth metal, e.g., yttrium or lanthanum, that is reactive withhydrogen to form when the rare earth metal thin film is contacted withhydrogen gas. The physical property change may comprise a change from ametallic state to a semiconducting state, whereby the step of monitoringthe physical property to determine the presence of hydrogen gas in theenvironment may be carried out by monitoring the electrical resistance,conductance, capacitance, or other electrical property of the rare earthmetal thin film.

[0101] The rare earth metal thin film in the broad practice of theinvention may suitably comprise at least one metal selected from thegroup consisting of: scandium, yttrium, lanthanum, cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, lutetium, actinium,thorium, protactinium, uranium, neptunium, plutonium, americium, curium,berkelium, californium, einsteinium, fermium, mendelevium, nobelium, andlawrencium, alloys thereof, and alloys containing one or more of suchmetals alloyed or doped with a suitable dopant component such as copper,cobalt, iridium, magnesium, calcium, barium, strontium,etc.

[0102] The hydrogen-permeable material of the protectivehydrogen-permeable barrier layer may suitably comprise a metal such asPd, Pt, Ir, Ag, Au, Co, Al and/or alloys thereof.

[0103] As a further variation, the hydrogen-permeable protective overlayer may be formed of alternating material layers. The material layersmay be formed of Pd, Ir and/or Pt.

[0104] As used herein, the term “thin films” will be understood asbroadly referring to films having a thickness of less than about 1,000microns.

[0105] In the use of hydrogen-interactive thin films in the practice ofthe invention for hydrogen sensing applications in which the thin filmwill or may encounter oxidizing species in the environment beingmonitored for hydrogen, such as oxygen, moisture (relative humidity),nitrogen oxides, carbon oxides, etc., it is advantageous to coat orencapsulate the hydrogen-interactive thin film with a hydrogen-permeableprotective material that prevents such oxidizing species, as well asother deleterious species in the environment, from contacting thehydrogen-interactive thin film.

[0106] The protective material may for example absorb oxygen and allowdiffusion of hydrogen through the protective material to the rare earthmetal thin film. Alternatively, the protective material may beimpermeable to oxygen and/or other oxidizing species.

[0107] The protective material when present as an over layer coating orencapsulant should be continuous and atomically dense in order toprovide an effective barrier against oxidation. The thickness of theover layer may be readily selected to minimize oxygen permeation whilemaximizing the response of the hydrogen-interactive thin film tohydrogen.

[0108] In one embodiment of the present invention in which a protectivematerial over layer is employed, the over layer may be formed of a metalsuch as Pd, Pt, Ir, or alloys or combinations thereof with one anotheror with other metal species. Particularly useful alloys for suchprotective material over layers include Pd-Ag (20%).

[0109] The CVD process when used to form the hydrogen-interactive thinfilm on the substrate, may employ bubbler delivery or liquid deliverywith subsequent flash vaporization, using a suitable precursor or sourcecompound, to generate a precursor vapor, which is transported to theheated micro-hotplate substrate for decomposition to form the desiredhydrogen-interactive film. Such precursors must be robust and volatileat the temperature of vaporization, yet they must decompose cleanly andefficiently on the substrate.

[0110] Particularly precursors for rare earth metal thin film formationby CVD in the practice of the invention includetris(cyclopentadienyl)lanthanum, tris(cyclopentadienyl)yttrium,β-ketoamine complexes of lanthanum, β-ketoamine complexes of yttrium,β-diiminate complexes of lanthanum, β-diiminate complexes of yttrium;lanthanum amides, and yttrium amides.

[0111] Suitable precursors may be readily determined within the skill ofthe art by screening techniques conventionally used in the art of CVDformation of thin films, including thermo gravimetric analysis (TGA) anddifferential scanning calorimetry (DSC) analysis.

[0112] For example, such simultaneous thermal analysis (STA) studiesunder argon and vacuum may be conducted to screen candidate precursorsfor suitable thermal stability and transport properties.

[0113] The STA studies are suitably conducted under conditionssimulating CVD conditions, e.g., under a flow of H₂ (5%) diluted withargon to provide data for predicting the major decomposition pathway(s)of the candidate precursors.

[0114] This combination of tests allows for rapid screening of a numberof potential precursors, and also allows the study of the effect ofother species present in the CVD process, e.g., reducing agents such asNH₃, on the decomposition pathway.

[0115] For example, hydrogen-interactive material thin films are fromabout 50 to about 2000 nm thick, more preferably from about 50 to about200 nm thick, with a protective layer when present having a thickness offrom about 2 to about 1000 nm, and more preferably from about 2 to about100 nm, e.g., a 20 nm thick protective layer of a material such as Pd ona rare earth metal thin film of 100 nm thickness. The protective overlayer is preferably thick enough to adequately protect the sensor fromoxidation and thin enough to leave unchanged the properties beingmonitored in the operation of the device.

[0116] The protective over layer may be deposited or formed over thehydrogen-interactive thin film in any suitable manner, includingspraying, solution deposition, dipping, chemical vapor deposition,physical vapor deposition, focused ion beam deposition, sputtering, etc.Generally, the methods described hereinabove for formation or coating ofthe hydrogen-interactive thin film in the first instance may also beused for forming the protective over layer thereon, and vice versa.

[0117] The protective over layer may be formed of any suitable materialof construction, which is suitably effective to prevent chemicalreaction or sorption processes from occurring that would preclude theefficacy of the hydrogen-interactive thin film for hydrogen sensing.

[0118] Although the protective over layer material is typically in theform of a film that is formed directly on the underlyinghydrogen-interactive thin film, it is possible within the broad scope ofthe present invention to utilize a protective material such as afree-standing film or a membrane that is in spaced relationship to thehydrogen-interactive thin film.

[0119] For example, the protective material may comprise a membrane thatis permselective for hydrogen only. The membrane may thus form a cellwithin which the hydrogen-interactive thin film is deployed.

[0120] The protective over layer material may for example be a metal, apolymeric film material, a vitreous or ceramic material, etc. Examplesof useful metals include Pd and other noble metals such as Pt, Ir, etc.

[0121] In the practice of the invention, Pd is utilized as a protectiveover layer material, and may be usefully deposited on thehydrogen-interactive thin film by chemical vapor deposition from acorresponding precursor.

[0122] Examples of precursors that may be used as source compositionsfor deposition of Pd by CVD include Pd(hfac)₂, Pd(allyl)₂ andCpPd(allyl).

[0123] In an aspect of the invention, the thickness of a Pd or othernoble metal protective over layer is selected to optimize the responseof the films to hydrogen. The over-layer is desirably continuous andatomically dense in order to provide an effective barrier againstoxygen. The thickness of the protective layer is strongly dependent onthe average roughness of the underlying film. The smoother thetopography of the underlying hydrogen-interactive, the thinner theprotective over layer can be to provide effective coverage.

[0124] Pd absorbs approximately nine hundred times its volume ofhydrogen gas. Although such absorption is reversible and highlyselective for hydrogen, excessive dissolution of hydrogen in the Pdprotective over layer may slow its diffusion to the underlyinghydrogen-interactive thin film. Such hydrogen dissolution may alsoresult in slow “re-zeroing” of the sensor after detection of hydrogen,due to slow rates of desorption, and the thermal actuation and output ofthe micro-hotplate are desirably utilized to compensate for the system“restoration delay” that would otherwise result in the absence ofthermal recovery operation by the micro-hotplate structure.

[0125] Both Pt and Ir absorb hydrogen and allow hydrogen to diffusethrough them and can readily be used in place of Pd. A number of Pd-richalloys also absorb hydrogen, e.g., Pd—Ag (20%). Membranes of this alloydo not undergo the volume expansion and cracking that is sometimesobserved for pure Pd and that may limit the use of such pure material.Pd-rich alloy membranes are used industrially and may be advantageouslyemployed in the broad practice of the present invention.

[0126] Rare earth metal alloys of magnesium are also useful as thehydrogen-interactive sense layer. The overall optical transmission rateof a rare earth-magnesium alloy hydride is higher than that of the puremetal hydride. The heat of formation of magnesium hydride (−33 kJ/mol H)is similar to that of rare earth hydrides (c.a. ˜40 kJ/mol H) and theuptake of hydrogen by these alloys is reversible. In addition, the bandgap of magnesium hydride is large enough that it forms a transparenthydride.

[0127] Alloying Gd with Mg to form the hydrogen-interactive sensinglayer yields a number of benefits. The alloyed films display much highertransmittance than pure Gd films. In Gd—Mg (30%) alloys maximumtransmittance is achieved at pressures well below 0.1 bar. Thischaracteristic makes the alloyed film very sensitive to hydrogen. Theslope of total transmittance vs. P[H₂] curve, below 1 bar, changesconsiderably with the concentration of Mg in the film. Alloying with asuitable metal, therefore, permits the sensory response of the film tobe selectively “engineered” for specific concentrations of hydrogen inthe product sensor device.

[0128] Alloying also increases the transmission ratio (i.e.,transmission of hydrided film/transmission of dehydrogenated film) toover 3000. This is due to the virtual elimination of all residualtransmission in the visible window. Residual transmission is typicallysmall (c.a. 1.5%) and of indeterminate origin. It is observed whensamples exposed to hydrogen are allowed to desorb in air. Alloying withmagnesium shifts the transmission window to shorter wavelengths whilegradually reducing the % transmission. For Gd-Mg alloy films containing30 at. % Mg, the maximum transmission of a 200 nm layer is 0.01%. Theseproperties make the Gd—Mg composition useful as an active layer materialto form a highly sensitive thin film sensor.

[0129] Alloys containing Mg at concentrations higher than 50 at. %exhibit three different optical states: transparent, absorbing, andreflecting; rather than just transparent and reflecting. Thisobservation can be exploited to provide another intermediate sensoryresponse, and enables the use of such alloys in tri-state opticalswitches.

[0130] The foregoing examples illustrate the utility of engineering theband gap and free energy of the rare earth dihydride to trihydridetransition, and such modification may be effected in the broad practiceof the invention by the addition to the hydrogen-interactive thin filmof a wide variety of potentially suitable dopants.

[0131] The specific dopant employed, and its concentration, areappropriately selected to enable the formation of an alloy hydride thathas a band gap large enough to be transparent in the visible region orotherwise appropriately constituted for a detectable change of propertyor properties in exposure to hydrogen. Ideally, the dopant will alsorender the dihydride to trihydride equilibrium thermodynamicallyneutral. Mg, Ca, Ba, Sr, Al, Ir and Co is potentially useful dopantspecies for such purpose. Transition metal elements such as Co and Irform a variety of stoichiometric and non-stoichiometric hydride speciesand may be particularly useful in a given end use application.

[0132] In one embodiment of the invention, the hydrogen-interactive thinfilm may be layered comprising one or more thin films wherein at leastone thin film is selected from the group consisting of rare earthmetals, Group II elements or any combination thereof. The rare earthmetal and the Group II element may be combined to form a Group IIelement doped rare earth metal thin film or an alloy thin filmcomprising the rare earth metal and the Group II element. Thisembodiment represents another technique for selectively varying theresponse characteristics of the hydrogen-selective thin film to achievea desired sensory sensitivity for the hydrogen-selective thin filmsensor.

[0133] Doping techniques are well known by those skilled in the art.Doping may include the addition of at least one element impurity to thehydrogen-interactive thin film or the deposition of a thin film adjacentto the hydrogen-interactive thin film so as to produce ahydrogen-interactive thin film with a desired characteristic.

[0134] In another embodiment of the invention, the protective over layeron the hydrogen-interactive thin film may be layered, with alternatingconstituent layers of over layer materials such as Pd, Ir, Rh, Ag, Au,Co, Pt and/or alloys thereof, as another technique for selectivelyvarying the response characteristics of the protective over layer toachieve a desired sensory sensitivity for the hydrogen-selective thinfilm sensor.

[0135] For example, a sensory Y and/or Gd film may be formed withalternating protective over layers of elements such as Pd and Pt, toprovide maximum sensitivity and capability over a wide range of hydrogenconcentration. The Pd/Pt interlayer in such a structure acts as hydrogenstorage layers as well as oxygen barrier layers, thereby enhancing thesensitivity of the film. Such a construction also allows reduction ofthe thickness of the top layer well below 50 Å.

[0136] In another embodiment of the invention, the hydrogen-interactivethin film sensor may comprise a multi-layered hydrogen-interactiveelement wherein, a first deposited thin film comprising Mg is depositedadjacent to the micro hotplate structure and a second thin filmcomprising Y is deposited adjacent to the first deposited Mg thin filmwherein, the multilayered hydrogen interactive element comprising afirst Mg thin film and a second Y thin film would be coated with a Pdprotective over layer.

[0137] In an embodiment of the invention, the hydrogen-interactive thinfilm sensor may comprise a hydrogen-interactive element wherein, a thinfilm of Y is deposited adjacent to the micro hotplate structure, and aPd protective over layer is deposited adjacent to the hydrogeninteractive element.

[0138] In another embodiment of the invention, the hydrogen-interactivethin film sensor may comprise a hydrogen-interactive element wherein, athin film consisting of 30 At % Mg and 70 At % Y is deposited adjacentto the micro hotplate structure, and a Pd protective over layer isdeposited adjacent to the hydrogen-interactive element.

[0139] The foregoing illustrative materials, Pd, Ir, Rh, Ag, Au, Co, Ptand/or alloys thereof, may be deposited to form the sensor device by anysuitable method, with CVD being generally . A wide variety of usefulprecursors for such CVD formation of the material on a given substrateor intermediate structure of the sensor may be readily determined withinthe skill of the art and without undue experimentation.

[0140] Examples of potentially useful precursors for Mg and Ir includeMg(thd)₂ and (COD)Ir(hfac), respectively.

[0141] Precursors for Al include, for example, the dimethylethyl amineadduct of alane (AlH₃) or dimethylaluminumhydride (DMAH), an airsensitive volatile liquid that is useful to deposit high qualityaluminum films.

[0142] Cobalt precursors include cobalt beta-diketonates such asCo(thd)₂ or Co(hfac)₂.

[0143] Referring now to the drawings, FIG. 4 is a scanning electronmicroscope (SEM) micrograph of a thin film sensor including a thin filmsensor element deposited by metal organic chemical vapor deposition(MOCVD) on a micro-hotplate structure. The micro-machined sensorplatforms define a 4-element gas-sensing array in which the activeelements are shown as light gray regions.

[0144]FIG. 5 is an exploded view of constituent layers of a hydrogensensor 10 of the type shown in FIG. 4, and constructed according to oneembodiment of the present invention. The lowermost layer 15 is formed ofsilicon dioxide (SiO₂) and defines a suspended membrane or micro bridge.The next succeeding layers include polycrystalline silicon heatingelement 16, silicon dioxide insulating layer 17, conductive heatdistribution plate 18 formed of aluminum, silicon dioxide insulatinglayer 20, four aluminum contact pads 22, and silicon dioxide insulatinglayer 24 with four openings therein communicating respectively with thefour aluminum contact pads 22. The layers 15, 16, 17, 18, 20, 22 and 24corporately constitute the micro-hotplate structure of the hydrogensensor.

[0145] Overlying the silicon dioxide insulating layer 24 is the thinfilm sensor layer 26. The thin film sensor layer 26 may comprise only arare earth metal thin film, or such rare earth metal thin film may beoptionally overlaid with a hydrogen-permeable protective barrier layerthin film.

[0146] The micro-hotplate structure of the hydrogen sensor shown in FIG.5 may be constructed as more fully described in U.S. Pat. No. 5,356,756to R. Cavicchi, et al. Typical physical characteristics are listed inTable 1 for the micro-hotplate structure of FIG. 5 comprising thethermally isolated, suspended resistive heater, the thin filmthermometer, and the four contact pads for measuring the conductance ofthe active layer. TABLE 2 Typical Micro-hotplate PhysicalCharacteristics Suspended Mass ˜0.2 □g Suspended Area 100 □m × 100 □m,Maximum Surface Temperature 550° C. Thermal rise time, fall time 1-3 ms,3-4 ms Continuous-use Power Consumption 60 mW

[0147]FIG. 5 is a schematic cross-sectional elevation view of a hydrogensensor 10 according to one embodiment of the present invention showingthe constituent layers of the structure on a silicon substrate 8. In theFIG. 5 device, elements corresponding to those of FIG. 4 arecorrespondingly numbered. In the device structure of FIG. 5, the silicondioxide layer 15 is overlaid in sequence by polycrystalline siliconheating element layer 16, silicon dioxide insulating layer 17,conductive (Al) heat distribution plate layer 18, silicon dioxideinsulating layer 20, Al contact pads 22, silicon dioxide insulatinglayer 24. The silicon substrate 8 is removed from the pit 9 therein,below the silicon dioxide, thus creating a suspended micro bridge. Thesuspended structure is overlaid with the thin film sensor layer 26,including a rare earth metal thin film optionally overlaid with ahydrogen-permeable protective barrier layer thin film to prevent oxygenand other oxidizing species from contacting the rare earth metal thinfilm.

[0148]FIG. 6 is a schematic representation of a hydrogen sensorapparatus 50 according to one embodiment of the invention. The hydrogensensor apparatus 50 includes a hydrogen sensor device 10 that may beconstructed and arranged as described hereinabove.

[0149] The hydrogen sensor device 10 is connected by signal transmissionline 48 to the central processor unit 44, which may comprisemicroprocessor or computer control elements for actuation, monitoringand control of the hydrogen sensor device. The central processor unit 44processes the signal carried by signal transmission line 48, andproduces an output signal that is transmitted in signal transmissionline 46 to output device 40, which produces an output that is indicativeof the presence or absence of hydrogen in the environment to which thesensor is exposed.

[0150] The output of the central processor unit 44 may include anyperceivable output, such as auditory output, visual output, tactileoutput (as for example when the hydrogen sensor apparatus is adapted tobe worn on the body of a user, and the central processor unit comprisesa vibrator imparting vibratory sensation to the user's body whenhydrogen is detected in the environment, such as may be useful inenvironments where auditory or visual outputs are not readilyperceivable.

[0151] In lieu of producing an output, which is perceivable, the centralprocessor unit 44 may be programmed to actuate means for eliminatinghydrogen from the environment being monitored, as for example a sweepgas flushing operation to purge the environment of the hydrogen gas.

[0152] It will be recognized that the hydrogen sensor may be constructedso that the rare earth metal thin film is arranged in hydrogenpermeation exposure to the environment being monitored. For example, theactive face of the sensor defined by the layer 26 in the FIGS. 5 and 6drawings may be contained in a sensing head which is insertable into aspecific gas environment susceptible to the incursion or in situgeneration of hydrogen therein.

[0153] The CPU 44 may be programmably arranged to maintain anappropriate monitoring status indicative of the presence or absence ofhydrogen gas in the environment being monitored. The CPU may include anelectrical resistivity monitor communicating by signal transmission line48 with the hydrogen sensor device 10, to monitor the change inelectrical resistivity of the film element incident to the introductionof hydrogen into contact with the hydrogen sensor device 10, and toresponsively generate a corresponding output signal.

[0154]FIG. 7 is a perspective view of a hand-held hydrogen sensor unit60 according to one embodiment of the present invention, comprising thesensor apparatus in housing adapted for manual transport and deployment.The sensor unit 60 may for example be constructed with an audible alarmindicating the presence of hydrogen gas in the environment beingmonitored. Such hydrogen sensor unit may be conveniently fabricated as asolid-state battery-powered device, with a very small weight.

[0155] It will be appreciated that the hydrogen sensor of the presentinvention may thus be provided in a wide variety of potentially usefulconfigurations, for a corresponding variety of hydrogen sensingapplications.

[0156]FIG. 8 is graph of the response of a H₂ sensor including a 15 nmthickness of palladium deposited on 300 nm of yttrium, overlaid on asuspended micro hotplate structure. The top panel of the graph shows themeasured resistance of the sensing film as a function of time, and thebottom panel of the graph shows how the concentration of H₂ was variedwith time. The testing was done at atmospheric pressure, in a nitrogenambient environment. The micro hotplate element was held at atemperature of ˜400° C. There is rapid increase in resistance when H₂ isintroduced to the sensor, and magnitude of the response increases withincreasing H₂ concentration.

[0157]FIG. 9 is graph of the response of a H₂ sensor including a 15 nmlayer of palladium deposited on 300 nm of yttrium, overlaid on asuspended micro hotplate structure, as a function on H₂ concentration.The testing was done at atmospheric pressure, in a nitrogen ambient andthe micro hotplate element was held at a temperature of ˜400° C. Theresponse of the sensor is approximately linear with respect to the logof the H₂ concentration over the range tested, viz., 0.1% to 4% H₂. Thecharacter of such response suggests that such range could readily beextended from 0.01 to 10% of the range that was tested, which is adynamic range of 3 orders of magnitude.

[0158] The measured response of these gas sensors is the change inresistance that occurs in the active layer film stack when exposed tohydrogen, where the resistance of the film increases with increasinghydrogen concentration. Based on the design flexibility of themicro-hotplate, the resistance of these films can be measured in eithera 2-wire or a 4-wire configuration.

[0159] Accurately measuring the speed of response to H₂ of the gassensors is an important design consideration for both the datacollection systems and the gas handling manifolds utilizing thesesensors. One embodiment of the present invention uses an automatedsystem for the data collection based on an HP 34970A DMM data loggerwith an HP 34902A scanning card. This system is capable of a scanningspeed of 250 channels/s. In order to achieve fast gas switching speeds,the gas handling manifold used low volume gas chromatography valves incombination with ⅛″ tubing and a small test chamber size. However, oneshould note that the present invention need not be limited to thisspecific embodiment or configuration.

[0160] The ambient gas used for the experiment was triple filteredcompressed air that was passed through a membrane drier, with a dewpointspecification of −40° C. Grade 5.0 hydrogen was used and blended withthe air using mass flow controllers with ranges of 200 and 5000 sccmrespectively.

[0161]FIG. 10 shows the resistive response of a micro-hotplate based H₂gas sensor. The measurement was made in a 2-wire configuration, and themicro-hotplate was held at an elevated temperature by passing current(<5 mA) through the embedded polysilicon heater. In this experiment, thesensor was cyclically exposed 10 times to 0.25% H₂ in dry air.

[0162]FIG. 11 focuses on the transition of one particular cycle with anexpanded scale. In FIG. 11, a rise time of <0.44 s was measured. Itshould also be noted that the magnitude of the response was greater than120%. This can be compared with the typical change in response ofpalladium alloy resistors, which is on the order of 10% when exposed to1 atm of H₂.

[0163]FIG. 12 shows the response of a micro hotplate to differentconcentrations of H₂. In this experiment, the initial concentration was1%, and it was decreased by a factor of 2 with each step until a finalconcentration of ˜0.01% (150 ppm) was reached. The sensor was exposedtwo times at each concentration. The exposure time was 300 s and thetime between exposures was also 300 s. The sensor exhibited detectableresponses to nearly two orders of magnitude of H₂ concentration. Thetemperature of the hotplate was not intentionally varied in thisexperiment. It seems likely that the minimum detectable gasconcentration can be further improved by optimizing the operationconditions at lower H₂ concentrations.

[0164]FIG. 13 is a plot of the responses from FIG. 12 as a function ofH₂ concentration. For this plot, the response was taken as the absolutechange in resistance as measured from the beginning base lineresistance. The H₂ concentration is plotted on a logarithmic axis, andshows that the response does not follow a simple dependence on the H₂concentration. The reasons for the behavior of the resistivity as afunction of H₂ are not currently well understood. One factor influencingthe behavior of the curve in FIGURE is the fact that at the lower H₂concentrations, the films response does not appear to have come toequilibrium within the exposure time. In addition to this, the influenceof contact resistance in a two probe configuration should be considered.Further testing is required to obtain a more accurate understanding ofthis behavior.

[0165] Stability is an important requirement of any type of sensor. Tobegin the investigation in this area, the resistance as a function oftime without H₂ exposure was examined for a period of several days indry air, as shown in the top panel of FIG. 13. There was no flow overthe sensor at this time. During the first day or so there is a smallsteady reduction in resistance, which eventually leveled out. This smalldrift was on the order of an ohm, which represents hydrogen in thesub-200 ppm range, and may either be due to outgassing from the sensingfilm, or from the chamber wall. After this, the resistance reachedsteady state, with a standard deviation of ˜0.05 ohms. This resulted ina signal to noise ratio of ˜1200 (average value/standard deviation). Themiddle panel of FIG. 14 shows the power consumed by the polysiliconheater element of the micro hotplate over the same time frame, which isexpected to representative of the operating temperature. There appearedto be cyclical variation in the power, which has a ˜24 hour period, i.e.a day/night difference. When the resistance is multiplied by the powerconsumed, which, to first order, compensates for temperature, thevariation appears much reduced. The signal to noise now increases tonearly 3000, and a jump in resistance on day 6, which was lost in thenoise, becomes noticeable.

[0166] The previously described MEMS based hydrogen gas sensors thatcouple a MEMS structure, a microhotplate, with a hydrogen sensitivecoating made of palladium capped yttrium dihydride. This device hasdemonstrated capabilities as a Hydrogen sensor, with sensitivity to aslittle as 200 ppm of H₂ in dry air, and speeds of response of less than0.5 s. However, there is a need to detect additional gases such has NH₃and sulfur-containing compounds. Methods, thin film materials, and andsystems for use with microhotplate based devices for these applicationsare described as follows:

[0167] There are several materials systems for detecting NH₃, vis a visa microhotplate structure. These include:

[0168] a) The first method is based on the use of the Pd coated metalhydride film system that has successfully been used for the detection ofhydrogen. This modifies the Pd/Metal Hydride film stack by adding anadditional NH₃ catalyst layer to the surface. This layer which would beonly on the order of a few monolayers or less, and then induces thedisassociation of the NH₃ into H₂ and N₂, whereby the H₂ would thendiffuse into the hydride layer and be detected as a resistance change.The catalyst may be selected from W, Pt, Rh, an alloy or combination ofthose or any other metal or material that lowers the disassociationtemperature of NH₃ to within the operating range of the microhotplate,<550° C.

[0169] Another approach is to use metal stable sulfides, such as Cu₂S,AgS, and AuS as the NH₃ sensitive layer. In this approach, thin films ofCu, Ag, Au or potential other nobel metal sulfides (Pt, Pd, or Ir) aredeposited on the microhotplate structure, and sulfides are formed viaexposure to H₂S. These sulfides are then expected to exhibit ameasurable change in resistance upon exposure to NH₃. This has beendemonstrated for CuS and is extended here in the other metals mentioned.This concept is also novel for use of these with thin films on amicrohotplate structure.

[0170] Yet another method is to use an acidic conducting polymer such aspolyaniline and/or polythiophenes. In these materials, the adsorption ofthe basic gas NH₃ would change the conductivity level of the polymerthin film. This polymer film could be made as a high surface areapolymer to increase the signal levels. It should be noted that theresponse times of the sensors demonstrated by Hirata et al are difficultto ascertain, but it is likely that substantial improvement is possibleby thermal activation using the Microhotplate structure. Thermalprofiles as a function of time would be chosen to avoid degrading thepolymer coating. It is expected that the recovery time on removal of NH₃would also be enhanced by thermal activation. Again the use of themicrohotplate for temperature control is new and novel, especially inconjunction with these thin film layers

[0171] The second method for detecting sulfides is based on the use ofultrathin metal films (<50 nm) of Rh, Ir and the like. These materialshave been chosen because they are inert, have very low energies of oxideformation, and are known to form sulfur complexes. In addition, films ofPt, and Pd may be used either alone or as alloying compounds. Othermaterials that can be induced through heating to reversibly form asulfide would also be candidates. Materials such as Ag, Cu and Au arecandidates for instance, because although they are known to form quitestable sulfides, the addition of a catalyst or an alloy form may lead toa reversible sulfur reaction. It is also possible that nano-sized grainswould be beneficial to the reversibility of sulfide formation, leadingto a more versatile sensor technology. Addition candidate metalsinclude, but are not limited to, Cd, Zn, Pb, Sb, and Bi.

[0172] Hydrogen containing gases such as CH₄, C₂H₆, acetone, methanoletc. are used in large variety of industrial applications ranging fromsemiconductor thin film processing to petroleum and polymermanufacturing. The combustible nature of many of these gases as well asthe always increasing need for improved process control makes thedetection and monitoring of these gases vitally important. Difficultieswith the sensors that are currently used to detect these gases are thatthey are not chemically specific, and often will have similar responsefor different gases. In addition, many of these sensors are combustionbased and rely on the presence of oxygen. It is desirable to have asensor that will be reproducible and specific to individual hydrogencontaining gases, and will be able to operate in environments withlittle to no oxygen present. It is also desirable to have a solid statesensor that has no moving parts, has a response time on the order ofseconds, would operate with minimum power consumption, does not requirefrequent calibration, and could be used in a hand held portableinstrument

[0173] This embodiment combines the use of a catalytic layer on top of arare earth hydride micro-hotplate gas sensor array. The rare earthhydride micro-hotplate sensor has been demonstrated to be extremelysensitive to hydrogen in an oxygen free environment, FIG. 1. The presentinvention seeks to exploit this sensitivity to hydrogen for thedetection of hydrogen containing compounds. Temperature sensitivecatalysis can be used to decompose hydrogen-containing compounds, thusreleasing molecular or atomic hydrogen. This can then be detected by therare earth hydride sensing films. By monitoring both the resistiveresponse of the hydride thin films and the reaction temperature,specific gases can be identified. There is also the possibility ofdetecting low concentrations of H₂O by looking for catalyticallydisassociated hydrogen with this method as well.

[0174] There are several potential detection schemes or embodiments ofthe present invention. The first is the deposition of non-continuouscatalytic islands (e.g. platinum) on top of the palladium/yttrium layersthat will act the hydrogen sensor. In this configuration, as shown inFIG. 15 hydrogen released from the parent molecule at the platinumsurface will be immediately available at the palladium surface fordetection. One potential draw back to this is that the detectiontemperature must be the same as the catalysis temperature, which may notnecessarily be simultaneously optimized.

[0175] An alternative approach is shown in FIG. 16, wherein thecatalytic reaction proceeds independently of the sensing by using anadjacent catalyticlly coated hotplate. In this approach, hydrogenreleased from the catalytic hotplate would diffuse or drift to thesensing hotplate, where it would be detected. In this way, each hotplatecould be operated at the optimum temperature required for both thedecomposition and detection. In addition, fabricating these adjacenthotplates without aluminum would allow temperatures as high as 800° C.to be used for catalysis. Currently fully functional hotplates withaluminum can not be operated above 500° C.

[0176] The present invention involves depositing Pd coated Y, La orother RE hydride films on the micro-hotplate structure, as well as thedeposition of catalytic layers. The film fabrication may be accomplishedeither by physical vapor deposition methods or by chemical vapordeposition methods. If CVD is used, then the possibility exists that theseparate heating of individual micro-hotplates can be used to develop aself-lithographic process.

[0177] One embodiment of the sensor fabrication would consist of thefollowing steps. The desired micro-hotplate array would be designed andlaid out, and might consist of 4, 8 or more individual elements. Thiswould then be fabricated in a commercial CMOS process using a facilitysuch as the MOSIS system. This would be micromachined and packaged. Thepackaged chip would be placed in either a PVD or a CVD chamber and thethin metallic films deposited on the hotplates. With the appropriateelectrical feedthroughs, the hotplates can be heated to improve theproperties of the metal film deposition. Also with the appropriateelectrical feedthroughs, the resistance of the deposited films can bemonitored in situ and used as feedback for the deposition process. Forexample when a certain conductance is reached, the film will be so thickand this conductance value can be used to stop the growth at thisdesired thickness. This will work for the RE and the Pd metallic overlayers as well as the catalytic layers.

[0178] Another embodiment would follow the same basic steps as abovewith the exception that a non-commercial process might be used tofabricate the micro-hotplate instead of the CMOS process. Suchnon-commercial process might substitute Pt or W for the Al metallizationtypically used.

[0179] In either embodiment, Pd, RE, and catalytic films of differentthickness within the same array can be used to cover a broader dynamicrange of detection. For example, a thin film of Pd can be used to detectlow concentrations of hydrogen. A thicker film of Pd can be used todetect higher concentrations, because it will be take more aconcentration driving force for the diffusion o hydrogen through thethicker layer.

[0180] In addition, in either embodiment, experiments can then beconducted to determine the optimal operation temperature ortemperatures. Because of the rapid thermal rise and fall time of themicro-hotplate, pulsed temperature operation can be considered. Forexample, the films might be most sensitive to initial hydrogen exposureat one temperature, but need a higher temperature to be returned totheir initial state. The sensor could then be pulsed periodically to berefreshed, thus minimizing the effect of drift and improving long termstability.

[0181] The features and advantages of the present invention are morefully shown by the following non-limiting examples.

EXAMPLE 1 Thin Film Deposition of Yttrium by Physical Vapor Deposition

[0182] Vacuum refined yttrium lumps (99.9%) and palladium pellets(99.9%) were melted in an electron beam PVD tool and used as targets.Depositions were carried out on polished, high grade, quartz photomaskblanks. A deposition methodology was established by trial and error thatensured the exclusion of oxygen and moisture in the deposition chamber.A 150 Å thick layer of Pd was determined to be necessary to protect thesensory yttrium layer.

[0183] An AFM topographical image of one of the films showed that theroot mean square (RMS) roughness of the Pd protective over layer was10.8 nm which was more than that of the film grown by CVD (2.5 nm). TheR_(max) of the film grown by PVD was also more than that of the filmgrown by CVD. Nevertheless, films grown by PVD are visibly smooth andreflective, in relation to the films grown by CVD.

EXAMPLE 2 Effects of Exposure of Rare Earth Metal Thin Films to Hydrogen

[0184] Strips of rare earth metal thin films were placed in a 1-inchdiameter quartz CVD tube and exposed to slightly less than oneatmosphere (700 Torr) of hydrogen. The color of the film turnedyellowish within 2-3 minutes, indicating the permanent conversion of Yto YH₂. Within a minute of this color change the film displayed astriking change in optical transmission, changing from opaque andreflective to transparent. This optical change is reversible andprovides a reversible hydrogen sensor. Upon removal of hydrogen animmediate loss of transparency was noted although complete opacity wasrestored after only several hours. This demonstrates the suitability ofrare earth metal thin films for inexpensive, hydrogen-specific, opticalsensors in accordance with the present invention.

EXAMPLE 3 Hydrogen Selectivity of Rare Earth Metal Thin Films

[0185] A series of film growth experiments was carried out to determinethe effect of film thickness both on stress and on the sensoryproperties of the film. Three sets of films (4 each) with yttriumthicknesses of 2500, 4000 and 5000 Å were grown. Each film had a 150 DPd protective over layer deposited thereon.

[0186] The selectivity of the sensor was demonstrated by optical changefrom opaque to clear when the films were exposed to:

[0187] 1) hydrogen diluted in 50% nitrogen;

[0188] 2) hydrogen-saturated pentane vapors, thereby presenting hydrogento the sensor in a low boiling organic solvent; and

[0189] 3) hydrogen diluted with 50% ammonia.

[0190] These results demonstrated the selectivity of the sensor of thepresent invention. We are unaware of any commercially available sensorthat can detect hydrogen under any of the above conditions (1)-(3).

EXAMPLE 4 Fabrication and Testing of Rare-Earth Coated Microhoplate H₂Gas Sensor

[0191] Micro hotplate structures were fabricated through a commercialfoundry and the as-received die was micro machined using XeF₂ as asilicon selective etchant. A photolithographic lift-off process was usedin combination with physical vapor deposition (PVD) to sequentiallydeposit yttrium thin films overlaid by palladium on the suspended microhotplate structures. Vacuum refined yttrium lumps (99.9%) and palladiumpellets (99.9%) were melted in an electron beam deposition tool and usedas targets. The EDS spectrum of the films clearly indicated the presenceof both yttrium and palladium on the microhotplates. These devices werewirebonded and packaged in 40 pin ceramic chip carriers.

[0192] The fully packaged chips were placed in a sealed chamber, andelectrical contact made via feed-throughs into the chamber. Nitrogen andhydrogen were introduced into the chamber and controlled with mass flowcontrollers and actuated valves. The resistance of the sensing film wasmeasured periodically with a digital multimeter and logged on a desktopcomputer. A DC power supply was used to heat the microhotplates. It wasfound that these devices have a significant resistive response tohydrogen in the absence of oxygen. Both the magnitude and speed of thisresponse was found to depend on temperature, thus indicating the valueof the micro-hotplate platform. Changes in resistance of greater than110% were observed in hydrogen concentrations of 3%. Extrapolation ofresponses measured over a decade of hydrogen concentrations, (0.1%-3%)suggests that better than 100 ppm sensitivity is achievable. The lowestrise and fall times measured were 30 and 300 s respectively.

[0193] A significant advantage provided by the present invention is thatthe micro hotplate based approach for H₂ gas sensing is based oncommercially available semiconductor processing technology. Thistechnology is readily accessible though a number of integrated chipfoundry facilities. The small size and simplicity of the micro-hotplatedevice can further leverage this advantage, and we therefore estimatethat more than 1 million devices could be produced on a single lot of 25six-inch wafers. Analysis further indicates that at these quantities,the final device cost becomes dominated by the packaging costs.

[0194] These sensors have shown exceptional responsivity. Changes inresistance of >120% to 0.25% H₂ concentrations have been measured, withresponse times <0.5 sec. These sensors have demonstrated a dynamic rangeof two orders of magnitude, detecting H₂ from >200 ppm to >1%. In thearea of stability, we have demonstrated an un-corrected baseline signalto noise ratio of ˜1200, and a temperature compensated signal to noiseof ˜3000. From a commercialization standpoint, our preliminary analysisindicates that this technology is readily scalable to quantities >1million devices. These results are extremely encouraging and suggestthat this technology has substantial potential for meeting the sensingrequirements of a hydrogen based energy economy.

[0195] The present invention provides another benefit, in that MEMSbased gas sensors may be produced via a CMOS foundry process. Severallaboratories have described the realization of such micro-machinedsuspended structures via a CMOS foundry process. First micro-hotplatedevice structures may be designed using available CAD layout softwarepackages and fabricated through foundry service. Next, as-received chipsare etched using XeF₂ or other known processes to create suspendedmicro-hotplate device structures. The functionalization step involvesapplying a H₂ sensitive coating to the surface of structures. Theprecise nature of both the materials and deposition can be thought of asa rare-earth based film, overcoated with a palladium-based layer. Thefinal fabrication step is to dice and package the solid-state sensors.

[0196] While the invention has been described herein with reference tovarious illustrative aspects, features and embodiments, it will berecognized that the invention is not thus limited, but ratherencompasses numerous other variations, modifications and otherembodiments, as will readily suggest themselves to those of ordinaryskill in the art, based on the disclosure and examples herein.Accordingly, the invention is to be broadly construed and interpreted,with respect to the ensuing claims, as including all such variations,modifications and other embodiments within its spirit and scope.

What is claimed is:
 1. A hydrogen sensor, comprising: at least onehydrogen-interactive thin film sensor element comprising a rare earthmetal or a rare metal dihydride; at least one micro-hotplate structurecoupled to said hydrogen-interactive sensor element for selectiveheating of the sensor element; and a hydrogen-permeable materialoverlaying each hydrogen-interactive element for selective permeation ofhydrogen, wherein said hydrogen-permeable material comprises an alloy tosuppress a phase change in said rare earth metal or rare metaldihydride.
 2. The hydrogen sensor of claim 1, wherein said alloy tosuppress a phase change in said rare earth metal or rare metal dihydridecomprises at least one element selected from the group of silver,titanium, nickel, chromium, aluminum.
 3. The hydrogen sensor of claim 1,wherein said alloy to suppress a phase change in said rare earth metalor rare metal dihydride comprises a rare metal oxide.
 4. The hydrogensensor of claim 1, wherein the rare earth metal or rare earth metaldihydride of the hydrogen-interactive sensor element, arranged forexposure to an environment susceptible to the incursion or generation ofhydrogen exhibits a detectable change of physical property, is exposedto hydrogen.
 5. The hydrogen sensor of claim 4, wherein said detectablechange of physical property is selected from the group consisting ofoptical transmissivity, electrical resistivity, electrical conductivity,electrical capacitance, magneto-resistance and photoconductivity.
 6. Thehydrogen sensor of claim 4, further comprising a detector constructedand arranged to convert said detectable change of physical property to aperceivable output selected from the group consisting of visual outputs,auditory outputs, tactile outputs, and auditory outputs.
 7. The hydrogensensor of claim 1, wherein the rare earth metal or rare earth metaldihydride of the hydrogen-interactive sensor element comprises at leastone rare earth metal component selected from the group consisting oftrivalent rare earth metals that react with hydrogen to form both metaldihydride and metal trihydride reaction products, wherein the metaldihydride and metal trihydride reaction products have differing physicalproperties.
 8. The hydrogen sensor of claim 1, wherein thehydrogen-interactive thin film sensor element comprises at least onethin film layer comprising one or more metals, present in elementalmetal form and/or in a dihydride thereof, wherein the metal is selectedfrom the group consisting of: magnesium, calcium, strontium, barium,scandium, yttrium, lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium, ytterbium, lutetium, actinium, thorium,protactinium, uranium, neptunium, plutonium, americium, curium,berkelium, californium, einsteinium, fermium, mendelevium, nobelium,lawrencium, and alloys thereof.
 9. The hydrogen sensor of claim 4,wherein said detectable change of physical property comprises a changeof electrical property when the hydrogen-interactive thin film sensorelement is contacted with hydrogen gas.
 10. The hydrogen sensor of claim1, further including a monitor operatively arranged in monitoringrelationship to the hydrogen-interactive thin film sensor element toprovide an output indicative of the presence of hydrogen.
 11. Thehydrogen sensor of claim 4, wherein said detectable change of physicalproperty comprises a change from a metallic state to a semiconductingstate.
 12. The hydrogen sensor of claim 1, further including anelectrical resistance monitor operatively arranged in monitoringrelationship to the hydrogen-interactive thin film sensor element toprovide an output indicative of the presence of hydrogen in anenvironment in contact with the hydrogen-interactive thin film sensorelement.
 13. The hydrogen sensor of claim 1, wherein thehydrogen-interactive thin film sensor element is formed of a materialconsisting essentially of rare earth metal dihydride of one or moretrivalent rare earth metals, wherein said rare earth metal dihydride isreversibly reactive with hydrogen to form corresponding metal trihydrideexhibiting a detectable change of physical properties.
 14. The hydrogensensor of claim 4, wherein the hydrogen-interactive thin film sensorelement comprises yttrium, and the physical property change comprises achange of electrical conductivity or resistivity when thehydrogen-interactive thin film sensor element is contacted with hydrogengas.
 15. The hydrogen sensor of claim 1, wherein the hydrogen-permeablematerial is selected from the group consisting of palladium, platinum,iridium, silver, gold, cobalt, and alloys thereof.
 16. The hydrogensensor of claim 1, wherein the hydrogen-interactive thin film sensorelement comprises a rare earth metal thin film that is doped with adopant.
 17. The hydrogen sensor of claim 14, wherein said dopant isselected from the group consisting of magnesium, calcium, strontium,barium, and any combination thereof.
 18. The hydrogen sensor of claim17, wherein said dopant is deposited on the hydrogen-interactive thinfilm.
 19. The hydrogen sensor of claim 1, wherein the micro-hotplatestructure is controlled by a predetermined time-temperature program forcyclic heating of the hydrogen-interactive thin film gas sensor elementby the micro-hotplate structure.
 20. The hydrogen sensor of claim 1,wherein said hydrogen-interactive thin film has a thickness of fromabout 50 to about 2000 nm.
 21. The hydrogen sensor of claim 1, whereinthe hydrogen-permeable material is in the form of a thin film.
 22. Thehydrogen sensor according to claim 1, wherein the hydrogen-permeablethin film has a thickness of from about 2 to about 1000 nm.
 23. Thehydrogen sensor according to claim 1, comprising a plurality ofhydrogen-I interactive thin films.
 24. The hydrogen sensor according toclaim 21, wherein at least two hydrogen-interactive thin film sensorelements are covered by hydrogen-permeable material of differentthickness.
 25. The hydrogen sensor according to claim 21, wherein atleast two hydrogen-interactive films are differing materials.
 26. Ahydrogen sensor device, comprising: A hydrogen-interactive thin filmsensor element comprising a rare earth metal and /or a rare earth metaldihydride; a micro-hotplate structure Coupled to saidhydrogen-interactive sensor element for selective heating of the sensorelement; a hydrogen-permeable material overlaying saidhydrogen-interactive sensor element for selective permeation ofhydrogen, wherein said hydrogen-permeable material comprises an alloy tosuppress a phase change in said rare earth metal or rare metaldihydride; and a detector coupled with said hydrogen-interactive sensorelement for sensing a detectable change of physical property of thesensor element on exposure to hydrogen and generating a correlativeoutput indicative of hydrogen presence.
 27. The hydrogen sensor of claim26, wherein said alloy to suppress a phase change in said rare earthmetal or rare metal dihydride comprises at least one element selectedfrom the group of silver, titanium, nickel, chromium, aluminum.
 28. Thehydrogen sensor of claim 26, wherein said alloy to suppress a phasechange in said rare earth metal or rare metal dihydride comprises a raremetal oxide.
 29. The hydrogen sensor device according to claim 26,further comprising a power supply for the device.
 30. The hydrogensensor device according to claim 29, wherein the power supply isconstructed and arranged for actuating the micro-hotplate structureduring and/or subsequent to sensing the detectable change of physicalproperty of the hydrogen-interactive thin film gas sensor element inexposure to hydrogen.
 31. The hydrogen sensor device according to claim29, wherein the power supply is constructed and arranged for energizingthe detector.
 32. The hydrogen sensor device of claim 26, wherein saiddetectable change of physical property is selected from the groupconsisting of optical transmissivity, electrical resistivity, electricalconductivity, electrical capacitance, magneto-resistance andphotoconductivity.
 33. The hydrogen sensor device of claim 26, whereinthe detector is constructed and arranged to convert said detectablechange of physical property to a perceivable output selected from thegroup consisting of visual outputs, auditory outputs, tactile outputs,and auditory outputs.
 34. The hydrogen sensor device of claim 29,wherein the rare earth metal or rare earth metal dihydride of thehydrogen-interactive sensor element comprises at least one rare earthmetal component, in an elemental metal form and/or in a metal dihydridethereof, selected from the group consisting of trivalent rare earthmetals that react with hydrogen to form both metal dihydride and metaltrihydride reaction products, wherein the metal dihydride and metaltrihydride reaction products have differing physical properties.
 35. Thehydrogen sensor device of claim 29, wherein the rare earth metal and/orrate earth metal dihydride of the hydrogen-interactive sensor elementcomprises one or more metals, in elemental metal form and/or in acorresponding metal dihydride, selected from the group consisting of:magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, actinium, thorium, protactinium, uranium, neptunium,plutonium, americium, curium, berkelium, californium, einsteinium,fermium, mendelevium, nobelium, lawrencium, and alloys containing one ormore of such metals.
 36. A method of fabricating a hydrogen sensor on asubstrate, comprising: constructing on the substrate a micro-hotplatestructure; and forming on the micro-hotplate structure ahydrogen-interactive thin film comprising a rare earth metal and/or arare earth metal dihydride that upon exposure to hydrogen exhibits adetectable change of at least one physical property, and wherein thehydrogen-interactive thin film is arranged to be heated by themicro-hotplate structure; and forming on the hydrogen-interactive thinfilm a protective over layer comprising a hydrogen-permeable materialfor selective permeation of hydrogen, wherein said hydrogen-permeablematerial comprises an alloy to suppress a phase change in said rareearth metal or rare metal dihydride.
 37. The method of claim 36, whereinsaid alloy to suppress a phase change in said rare earth metal or raremetal dihydride comprises at least one element selected from the groupof silver, titanium, nickel, chromium, aluminum.
 38. The method of claim36, wherein said alloy to suppress a phase change in said rare earthmetal or rare metal dihydride comprises a rare metal oxide.
 39. Themethod of claim 36, further comprising coupling the hydrogen-interactivethin film with a detector for outputting the detectable change ofphysical property of the hydrogen-interactive thin film when thehydrogen-interactive thin film is exposed to hydrogen.
 40. The method ofclaim 36, wherein the hydrogen-interactive thin film comprises a rareearth metal component, in elemental metal form and/or in a correspondingmetal dihydride, selected from the group consisting of trivalent rareearth metals that react with hydrogen to form both metal dihydride andmetal trihydride reaction products, wherein the metal dihydride andmetal trihydride reaction products have differing physical properties.41. The method of claim 36, wherein the hydrogen-interactive thin filmcomprises one or more metal components, in elemental metal form and/orin a corresponding metal dihydride selected from the group consistingof: magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,lutetium, actinium, thorium, protactinium, uranium, neptunium,plutonium, americium, curium, berkelium, californium, einsteinium,fermium, mendelevium, nobelium, lawrencium, and alloys containing one ormore of such metals.
 42. The method of claim 36, further comprisingcoupling the hydrogen-interactive thin film with an electricalresistance monitor to provide an output indicative of the presence ofhydrogen in an environment in contact with the rare earth metal thinfilm.
 43. The method of claim 36, wherein the hydrogen-permeablematerial comprises a metal selected from the group consisting ofpalladium, platinum, iridium, silver, gold, cobalt, and alloys thereof.44. The method of claim 36, wherein the hydrogen-interactive thin filmcomprises a metal selected from the group consisting of lanthanum andyttrium, and the hydrogen-interactive thin film is formed on thesubstrate by chemical vapor deposition utilizing a correspondingprecursor, wherein said precursor is selected from the group consistingof tris(cyclopentadienyl)lanthanum, tris(cyclopentadienyl)yttrium,β-ketoamine complexes of lanthanum, β-ketoamine complexes of yttrium,β-diketonate complexes of lanthanum, β-diketonate complexes of yttrium,β-diiminate complexes of lanthanum, β-diiminate complexes of yttrium;lanthanum amides, and yttrium amides.
 45. The method of claim 34,wherein the hydrogen-interactive thin film is doped with a dopant. 46.The method of claim 42, wherein the dopant is deposited on thehydrogen-interactive thin film from a precursor, and said precursor isselected from the group consisting of Mg(thd)₂, Ca(thd)₂, dimethylaluminumhydride, Ba(thd)₂, Sr(thd)₂, (COD)Ir(hfac) and Co(thd)₂.
 47. Themethod of claim 36, wherein the hydrogen-interactive thin film comprisesyttrium, formed on the substrate by chemical vapor deposition utilizingas a precursor Y(NSiMe₃)₃.